In the case of such electric motors, more particularly ones of small size, it is frequently desired to have an electrical brake so that when the electric motor is turned off its rotor quickly comes to a standstill. Taking the use of such electric motors in home power tools as an example, safety requirements call for some provision to ensure that the electric motor stops rotating promptly when it has been turned off. In fact, it is frequently the case that a power tool bit such as a saw blade or a grinding wheel of a grinding tool, projects from the rest of the tool so that the same may only be put down when the electric motor has completely stopped moving. This is a further reason calling for a prompt braking effect.
In the case of one known brake device, the remanence field of the iron circuit of the electric motor is utilized to trigger braking excitation, the rotor of the motor being short circuited for braking. However, in the case of the known brake device, the ac operation means that the remanence field of the iron circuit is not always the same. The principle of operation further means that the braking action is not reproducible, because the remanence field initially causing the excitation is determined by the instant of turning off and it may be too small for the initiation of the braking operation. Furthermore, the braking torque may only be built up if the direction of the rotor or field current is reversed. If the initiating remanence field is sufficient, the build up of the field is produced with a brake current so that without regulation of the brake current high braking currents have to be commutated, this leading to sparking at the brushes and, thus too high wear of the brushes and of the commutator. In order to prevent this, it is necessary to provide an elaborate and expensive brake current regulating system.
Furthermore, a device for braking such electric motors is described in the German patent publication 3,636,555, in which the initiation of the brake regulation is performed electrically or, respectively, electronically. In the case of this known brake device as well, reversal of the field winding polarity is necessary. The initiating braking field is not constant, because the initiating electrical field, which is produced by a capacitor current, is superimposed on the remanence field so that the braking time is not regular and constant. The means for initiating and regulating the excitation is very elaborate for which reason an additional electronic assembly is necessary. The necessary power handling capacity of the excitation circuit makes it elaborate and the brake energy storage means, preferably in the form of electrolytic capacitors, has only a limited working life. In the event of failure of components of the excitation circuit, braking is no longer possible. The braking current has to be regulated in addition in order to have a predictable braking time and to check sparking at the brushes. The braking energy is converted into heat in a braking resistor so that the number of the braking operations possible in succession is limited or they make an additional cooling of the components necessary.